Good news for Buckingham Palace, bad news for Kate Middleton: the Duchess of Cambridge and Prince William are expecting their third child.

The news that Kate is expecting her new bundle of joy clearly has Great Britain delighted. But there are many who are concerned with both her and her baby's well-being. The Duchess has suffered from hyperemesis gravidarum (HG), a serious and mysterious medical condition that includes dehydration, nausea, vomiting, and weight loss. This could put both Kate and her baby in great danger.

Kate had to cancel a visit to a clinic to discuss the mental health involved with giving birth. This condition doesn't have a certain number of diagnoses to reference, which makes it an even bigger problem, according to NBC News. HG runs on a spectrum with no distinct definition. Between 0.5 percent and three percent of pregnant women suffer from HG, according to Sara Twogood, assistant professor of clinical obstetrics and gynecology at Keck Medicine of USC. HG accounts for over 167,000 ER visits each year.

"Different clinicians use different criteria to meet the definition of [HG]," says Twogood. "There's really no strict definition."

Some women may have HG and not know it, while others may suffer from severe nausea and vomiting. Sometimes, the latter are seen as common signs of early pregnancy. Women may not know that what they're experiencing are not normal signs of what to experience when they're expecting.

"Twenty-five percent of pregnant women will have nausea and 50 percent of women will experience nausea and vomiting," Twogood says.

Even most physicians overlook the symptoms of HG. The symptoms of hyperemesis gravidarum are complex since a clear definition is hard to pinpoint. Some of the symptoms are swept under the rug. According to Twogood, the common symptoms of HG include severe nausea, persistent vomiting, loss of appetite, and extreme weight loss.

"If you lose five percent of your pre-pregnancy body weight or more it is considered HG," said Dr. Marlena Fejzo, associate faculty researcher at UCLA and USC. "This is usually accompanied with an electrolyte imbalance and dehydration."

Unfortunately, there is no cure for hyperemesis gravidarum. No one knows what exactly it's caused by.

"I am a geneticist by training and this is what I've been working on," said Fejzo. "We know it runs in families, and that if you have a sister who has had it you have a 17-fold increase risk of having it."

If you had it before, you will likely experience it in future pregnancies. This is exactly what happened to Kate. She had a severe case of morning sickness when she was pregnant with her child Prince George. During her second pregnancy with Princess Charlotte, it wasn't as severe. However, she is suffering from hyperemesis gravidarum once again. She wasn't able to attend Prince George's first day of school.

Kate has been open about her struggles with pre-pregnancy and post-pregnancy in the past. She recently revealed that she suffered from post-partum depression and has questioned her parenting skills. There are support groups available to help women cope with hyperemesis gravidarum just as there are for those suffering from post-partum depression. Some of these support groups are available in local areas or online.